Reinforcement bar and method for manufacturing same

ABSTRACT

Reinforcement bars for concrete structures, comprising continuous, parallel fibers, made of basalt, carbon, glass fiber, or the like, embedded in a cured matrix, each bar being made of at least one fiber bundle comprising a number of parallel, cylindrical cross section fibers and said bars being provided with a surface shape and/or texture which contributes to good bonding with the concrete. Part of the surface of each bar being deformed prior to or during the curing by: a) strings of an elastic or inelastic, and/or b) at least one deformed section of each reinforcement bar; thereby producing a roughened surface.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a National Stage of International ApplicationNo. PCT/N02011/000300 filed on Oct. 21, 2011, which claims the benefitof Norwegian Patent Application No. 20101485 filed on Oct. 21, 2010. Theentire disclosures of which are incorporated herein by reference.

THE TECHNICAL FIELD OF THE INVENTION

The present invention relates to a reinforcement element for use inconnection with structures to be cast, such as for example concretestructures.

More specifically, the present invention relates to reinforcement barsfor concrete structures and a method for manufacturing such bars,comprising a large number of continuous, parallel fibers, slightlytensioned to work together, preferably made of basalt, carbon, glassfiber, or the like, embedded in a cured matrix, the bars preferablyhaving an average length of 20 mm to 200 mm, and an average diameter of0.3 mm to 3 mm, each bar being made of at least one fiber bundlecomprising a number of parallel, preferably straight fibers having acylindrical or oval cross section and said bars being provided with asurface shape and/or texture for bonding properties.

BACKGROUND FOR THE INVENTION

Plain concrete is strong in compression, but is very weak in tension,resulting in low tensile strain failure. Therefore, it is an establishedpractice to add small length fibers to concrete at the time of mixingthe ingredients of concrete. The fiber mixed with the concrete duringthe mixing will disperse in all directions in a random manner andprovide reinforcing effect in all directions within the cured, hardenedconcrete. Addition of fiber will change the cracking mode from macrocracking to micro cracking. By modifying the cracking mechanism, themacro cracks become micro cracks. Crack widths are reduced and theultimate tensile cracking strains of the concrete are increased. Themechanical bond between the embedded fiber and binder matrix providesfor this redistribution of stresses. Additionally, the ability to modifycracking mode results in quantifiable benefits, reducing micro crackingthat leads to reduced permeability and increased surface abrasionresistance, impact resistance and fatigue strength. This type ofconcrete is known as fiber reinforced concrete.

The use of corrosion resistant fiber reinforced polymer (FRP)reinforcement has also previously been proposed for transportationstructures particularly those exposed to deicing salts, and/or locatedin highly corrosive environment. Glass, carbon and aramid fibers arecommonly used in the manufacture of reinforcing bars for such concreteapplications.

Recent developments in fiber production technology allow the making ofbasalt fiber reinforcement polymer bars (BFRP), manufactured from basaltfiber which is made from basalt rock. Basalt fibers have good range ofthermal performance, high tensile strength, resistance to acids, goodelectro-magnetic properties, inert nature, resistance to corrosion,radiation and UV light, vibration and impact loading. BFRP products areavailable in a variety of forms, such as, straight rods, loops,two-dimensional mesh, and spirals.

Other areas of use of fibers for reinforcing structures are concretelayers or linings to be used on tunnel walls, either to prevent rockfrom falling down or as a fire preventing means. Such concrete are shoton to the surface and is commonly denoted as gunite or shotcrete as wellas precast concrete slabs or pre-fabricated concrete elements.

In order to prevent the consequential effects of creep during the curingstages, i.e. to prevent formation of minute or larger cracks during thecuring stage, fibers have been used. One type of fibers used is steelfibers having a length in the region of 2-5 cm and a diameter ofapproximately 1 mm. In order to provide sufficient bonding with theconcrete, the ends of such fibers are made flat, thus providing extendedheads. The purpose of said steel fiber reinforcement is to preventcracking during the curing stage of the green concrete.

Also fiber reinforcements made of a large number of parallel glass,aramid or carbon fibers embedded in a matrix and cured have previouslybeen proposed used instead of or in addition to steel fibers.

GB 2 175 364 A relates to a reinforcement member in the form of long,straight, elongate continuous reinforcement rods or bars, having atleast one projection on its surface, which is formed by wrapping acordlike material on the circumferential surface of a fiber-reinforcedsynthetic core. The cord-like material is formed by twisting continuousfiber bundles at a pitch in the range of three turns per ten cm tofifteen turns per ten cm. The fiber bundles comprise glass, or carbon,or boron, or metal, or natural or synthetic fibers.

U.S. Pat. No. 5,182,064 describes a method for producing a long,elongate fiber reinforced plastic rod having ribs on its surface byimpregnating a reinforcing material which has continuous long fiberbundles with an uncured liquid resin. A rib forming member is separatelyprepared by impregnating a fiber bundle reinforcement material with anuncured liquid resin. A fiber reinforced plastic rod is formed byhelically applying the rib forming member and together curing the twomembers into an integral body.

JP 4224154 describes a reinforcing member for concrete having highrestraining adhesion strength and tensile strength by winding thickthreads and fine threads around a core material comprising reinforcingfiber and thermosetting resin, and hardened and cured while forming arugged coating layer with a thermo setting.

JP describes how to improve the reinforcing strength of cement byforming outwards projecting ring shaped projections, or flatten ends, onelongate fiber bundles, embedded in a very sticky material, cuttingthese into short fiber bundles arranged in one direction and embedded ina resin matrix.

JP 1207552 describes a solution where a thermoplastic resin isreinforced with bundles of reinforcing fiber oriented in one direction,and thereto a bending process is applied. Where bending process is to beapplied, a yarn consisting of the same fiber as the mentionedreinforcing fiber is wound round, and powder consisting of siliconcarbide, aluminum oxide, stainless steel, etc., with rich attachingproperty to concrete is affixed to the peripheries of rod in order toincrease the attachment strength of a reinforcing member to theconcrete.

CN 2740607 discloses a reinforced fiber structure for concrete. Thefiber is a high polymer fiber which is provided with a rough surface.The cross-sectional shape of the reinforced fiber structure can be asix-leaf shape or a five-leaf shape. A profile shape may be a wave shapeor a saw-tooth shape. The diameter of the fiber is between 0.5 mm and1.0 mm. The length of the fiber is between 40 mm and 75 mm. The fiberstructure has high tensile strength, low elastic modulus, strong acidityand alkalinity resistance and a light specific gravity. The fiber isused for controlling cracks in the concrete during the curing stage.

CN 201236420 discloses rib material that can be used in constructioninstead of reinforcing steel bars. The fiber composite rib material is abendable cylindrical sectional bar formed by gluing and compounding aplurality of basalt fiber core bundles and a resin substrate coating thebasalt fiber core bundle. The bars are long units of a similar size asconventional reinforcement bars of steel.

EP 2087987 discloses a method and a device for introducing longer steelfibers in concrete using a device mounted on or close to a concretenozzle, where the fibers are cut and shot into the concrete flow througha pipe, directly into the concrete mixer.

JP2007070204 and JP 2008037680 describe a carbon fiber bundle in theform of a piled yarn of two or more carbon fiber bundles. The carbonfiber bundle is twisted 50-120 per meter and has a length in the orderof 5-50 mm. The carbon fiber bundle surface has corrugated interval of3-25 mm. The flat carbon fiber bundle having width/thickness ration of20 or more, is twisted and processed. The cross sectional are of thewire is 0.15-3 mm.

WO 98/10159 describes fibers, continuous or discontinuous, and barshaving optimized geometries for use in reinforcement of cement, thecross-sectional are of which is polygonal. The geometries are designedto increase the ratio of surface area available for bond between thefiber and the matrix to the cross-sectional area of the fiber

US 2001/0051266 and US 2004/0018358 describe fibers which aremicro-mechanically-deformed such that the fibers are flattened and havesurface deformations for improved contact with the matrix material, thematrix material inter alia may be concrete. The fibers have preferably alength in the region 5-100 mm and an average width of 0.5-8 mm, thefibers being made of one or more synthetic polymers or metal, such assteel.

WO 02/06607 describes fibers to be used in concrete mixtures, the fibersbeing flat or flattened and have a first and second opposed flat orflattened end that are twisted out of phase and which define therebetween an intermediate elongate, helical fiber body. The fibers have anaverage length of about 5-100 mm and average width of 0.25-8.0 mm andaverage thickness of 0.00-3.0 mm. The fibers are made of polypropyleneor polyethylene.

Reference is also made to WO 20093/025305, belonging to the applicant,such publication being included by the reference both with respect tothe method of fabrication and to the configuration and built-up ofelongated composite reinforcement bars.

It is a need for an improved type of reinforcement which in a simplemanner is suitable for repairing conventional cracked concretestructures, reinforced with conventional steel reinforcement such thatexposed steel reinforcement may be sealed off and in addition restoreand possibly providing added structural integrity of the crackedconcrete structure.

It is further a need for providing reinforcement for concrete structuresavoiding the need of complex or conventional reinforcement placed insitu, basing the reinforcement on more or less randomly placedreinforcement within the green concrete, reducing the requirement of orat least part of the conventional reinforcement.

Further, there is a need for an effective and improved method forproducing the short fiber bars and for improving the bonding effectbetween the surrounding concrete and the short bars.

There is also a need for a short bar reinforcement which contributes tothe concrete strength also at the stages subsequent to completed curingof the concrete.

It should also be appreciated that there is a need for a reliable,maintenance free reinforcement where access is limited for installationof bar reinforcement or for use in processes where the automatedmachinery limits the opportunity to use straight bar reinforcement orprefabricated or in situ placed reinforcement cages, includingstructures such as slabs, pipes, drainage culvers, pavement, seaanchors, etc.

In most of the documents referred to above, the plastic fibers used ischosen from a group having a specific weight contributing to a totalspecific weight of the fibers, i.e. fiber and matrix, which is less than1, thus giving short bars a tendency to float up towards the uppersurface in the pouring process. Further, plastic fibers of the prior arthave also a tendency to absorb water, causing dehydration in a castingphase where there is a need for a surplus of water to achieve a propercuring of the concrete.

When pouring the concrete, the prior art plastic fibers have a tendencyto float up towards the surface when leaving the chute. Further, theconventional steel fibers have a tendency to ball up during mixing andpouring, resulting in clogging, and is also hard to mix due to the waterabsorbance tendencies, having a negative effect on the dehydration andcuring process of the poured concrete. These negative effects reduce therange of volume fraction steel and plastic fiber can be used across. Theadvantage of the basalt MiniBars™ according to the present invention, isthe density and the non-water absorption, allowing mixing in ranges upto 10% volume fraction (VF), which otherwise would have been impossibleusing conventional fibers.

SUMMARY OF THE INVENTION

A key object of the present invention is to increase the tensilestrength of fiber reinforced concrete upwards to 15 MPa in flexuraltensile strength using ASTM Testing Methods and also residual tensilestrength, and to transform the compressive failure mode to plasticversus brittle, reducing the volume fracture to preferably below 10,thus establishing a very efficient reinforcement.

It is also an object of the present invention to provide a MiniBar™reinforced concrete having very good flexure toughness and energyabsorption capabilities after cracking. The definition of MiniBar™comprising short basalt, carbon or glass fiber reinforcement bars,formed of a number or substantially parallel fibers embedded in asuitable matrix, and comprising a helix wound around the embeddedfibers, forming helically arranged indents extending circumferentiallyin a continuous manner along bar, the bar having with a length in theregion of 20 to 200 mm and a diameter in the region of 0.3 mm to 3 mmand possibly with a roughened surface as further referred to below,hereinafter referred to MiniBar™.

Another object of the present invention is to provide a reinforcementbeing active both during the curing stage as inherent crack control andduring the life of the concrete structure, having load bearing anddistributing properties also subsequent to completed curing, thusimproving the structural integrity of such concrete structures.

Another object of the present invention is to provide a reinforcementelement which reduces the extent of preparatory work on damaged concretestructures in order to repair damages on such structures.

Another object of the present invention is to provide a method forproducing such bar reinforcement with enhanced bonding qualities andproperties when used in concrete.

Another object of the present invention is to provide a reinforcementsystem that also may be used in concrete structures such as sea wallswhere the improved concrete strength in tension would eliminate the needfor the light or moderate steel or other type of reinforcement.

Another object of the present invention is to provide a FRPreinforcement consisting of short bars which do not contribute in anegative manner to the curing process of the concrete while at the sametime enhancing the bonding effect and bonding mechanism with thesurrounding concrete.

It should be appreciated that steel fibers will due to its lack ofcorrosion resistance gradually lose its reinforcing strength. Hence,another object of the present invention is to provide an alkaliresistant reinforcement fiber.

A still further object is to provide a MiniBar™ reinforcement whichallows for random placing in the mix and which is not influenced by useof vibrators for vibrating the green concrete.

A further object of the present invention is to provide a reinforcementwhich is suitable for reinforcing structures which are otherwisedifficult to access, such as deep foundation in excavation, foundationpiles or diaphragm walls.

Another object of the invention is to provide a MiniBar™ reinforcement,the position of which is not affected when the green concrete isvibrated due to the density.

Another object of the present invention is to provide a reinforcementsystem where the reinforcing effect of the fibers and conventionalreinforcement in the form of reinforcement bars or loops work togetherover the entire cross sectional area of a concrete structure, and alsopreventing formation of cracks of the concrete and/or surface spallingsubsequent to completed curing of the concrete. In such case the fiberreinforcement and the reinforcement in the forms of bars, loops orpre-stressing reinforcement function as an integrated reinforcement.

Another object of the present invention is to provide a reinforcementsystem reducing the required labor cost and maintaining a feasible levelof workability of the green concrete.

Yet another object of the present invention is to provide reinforcementelements which are configured in such way that when a concretestructure, reinforced with the reinforcement elements according to theinvention is subjected to loads and forces, the failure shall be by lossof bonding between a reinforcement element and not by breaking theMiniBar™, allowing the concrete to fail or crack but not the MiniBar™itself, thus giving the concrete structure post cracking strengthrelated to the good bond strength.

Yet another object of the present invention is to provide improved,short bars which do not clog during mixing with green concrete and whichdo not sink or float up in a mixed, green concrete batch during mixingor pouring.

The objects are achieved by use of short MiniBar™ reinforcement asfurther defined by the independent claims. Possible embodiments aredefined by the dependent claims.

Yet another object of the present invention is to provide MiniBbar™reinforcement where the diameter and the bond strength, which arecritical dimensions for obtaining the strength, are combined in such waythat the required flexural and residual tensile strength exceeds 15 MPa.

According to the present invention the MiniBars™ also are intended toeliminate the need for steel or basalt fiber reinforcement polymers insome applications, such as shear reinforcement.

The above objects are achieved by a reinforcement bar and a method forusing and producing such bars as further defined by the independentclaims. Optional embodiments of the invention are defined by thedependent claims.

According to the present invention, the reinforcement bar for concretestructures, comprises a large number of continuous, parallel fibers,preferably made of basalt, carbon, glass fiber, or the like, embedded ina cured matrix. The bars may preferably have an average length in therange of 20 mm to 200 mm, and an average diameter in the range of 0.3 mmto 3 mm and each bar may be made of at least one fiber bundle comprisinga number of parallel, preferably straight fibers having a cylindricalcross section, the cross section preferably being more or less circularor oval. At least a part of the surface of each bar may be deformedprior to or during the curing stage of the matrix by means of:

a) one or more strings of an elastic or inelastic, but tensionedmaterial being helically wound around said at least one bundle ofparallel, straight fibers prior to curing of the matrix in which thefibers are embedded, maintaining the fibers in an parallel state duringcuring and providing an uneven external surface with longitudinallyarranged helical indents in a longitudinal direction on the surface ofthe matrixed fiber bundle(s) of the reinforcement bars, and/or

b) that said bars being provided with a surface shape and/or texturewhich contributes to good bonding with the concrete; thereby providing aroughened surface.

According to one embodiment of the invention, said two or more stringsmay be helically wound in opposite direction around the matrix embeddedfiber bundle(s).

Further, the mini bars may preferably be made of basalt fibers, carbon,glass or the like.

It should be appreciated that the pitch length of the helix is in therange of 10 mm to 22 mm, and preferably be around 17 mm to be matchedwith grade of concrete and aggregate size, while the angle of the helixwith respect to the center line of the mini bar fiber may preferably bein the range from 4 to 8 degrees, while the angle of the parallel fiberswith respect to said center line of the mini bar fiber should be between2 and 5 degrees.

The invention comprises also a method for manufacturing reinforcementbars. Each bar may comprise a large number of continuous, parallelfibers, preferably made of basalt, carbon, glass fiber, or the like,embedded in a cured matrix, the bars preferably having a length in therange 20 mm to 200 mm, and a diameter in the range of 0.3 mm to 3 mm.Said bars may be made of at least one fiber bundle, which prior to orduring the curing process are provided with a surface texturecontributing to good bonding with the concrete, said surface texture isobtained by helically winding one or more strings of an elastic materialaround said at least one bundle of parallel, fibers the fibers alsobeing straight.

According to one embodiment, at least one helically string is woundprior to curing of the matrix, holding the fibers in an parallel stateduring curing and providing an uneven external surface in a longitudinaldirection of the reinforcement bars. Two or more such strings may beused, for example wound helically in opposite direction.

The helical winding may be wound with an angle in the range of 4 to 8degrees, with respect to the center line of the elongate mini bar.

Such fibers may be randomly mixed with green concrete and used forrepair work of cracked concrete and also for providing average residualstrength and flexural strength in the cured concrete structures, therebyrestoring or improving the structural integrity of the concretestructure.

According to one embodiment of the invention, the reinforcement barcomprises a large number of continuous, parallel fibers, preferably madeof basalt, embedded in a cured matrix, the bars preferably having anaverage length in the range of 20 mm to 200 mm, and an average diameterin the range of 0.3 mm to 3 mm. Each bar may be made of at least onefiber bundles comprising a number of parallel, preferably straightfibers having a more or less cylindrical or oval cross section and beingprovided with a surface shape and/or texture which contributes to goodbonding with the concrete.

At least a part of the surface of each bar being deformed prior to orduring the curing stage of the matrix by means of:

a) one or more strings of a string material being helically wound aroundsaid at least one bundle of parallel, straight fibers prior to curing ofthe matrix in which the fibers are embedded, maintaining the fibers inan parallel state during curing and providing an uneven external surfacein a longitudinal direction of the reinforcement bars, and/or

b) at least one deformed section and/or possibly at least one end ofeach reinforcement bar; thereby producing a roughened surface and/orsuch deformations may be any deformations or dents or shapes preventingor at least substantially restricting pull out.

It should also be appreciated that a thinner basalt fiber used as helixaround the main basalt fiber bar will increase the strength of theMiniBar™.

According to a further embodiment, one, two or more strings arehelically wound in opposite direction, said one or more strings creatingthe indentations required according to the present invention.

According to the present invention, said helically arranged indents areprovided by twisting a thread or fiber unit helically around the bundleof impregnated, more or less uncured fibers, applying a higher tensionin said thread than in the bundle, thereby providing a twist in thebundle and/or a helically arranged indent extending along the entirelength of the bundle and/or the short cut-off bars as the case may be.

Alternatively or in addition, the exterior surface of the bar may beprovided with at least one enlarged or flatten portion or having varyingdiameter, such surface being provided prior to the curing phase, therebyproviding a better bonding with the concrete.

Each bar may also have a deformed middle section or ends, increasing thecontacting surface area of bar.

In a preferred method for manufacturing reinforcement bars as furtherdefined above said surface texture is obtained by helically winding oneor more strings of an elastic or inelastic material around said at leastone bundle of parallel, fibers the fibers also being straight. At leastone helically string may preferably be wound around the fibers andmatrix prior to curing of the matrix, holding the fibers in an parallelstate during curing and providing an uneven external surface in the formof helically extending indents in a longitudinal direction of thereinforcement bars. Alternatively, two or more strings may be helicallywound around the fibers and matrix in opposite directions, the tensionin such string(s) being higher than the tension used for pulling thebundle along the production line towards the curing and hardening stage.

The exterior surface of the bar may further or instead be provided withat least one enlarged or flatten portion or having varying diameter,such enlarged or flatten portion being formed prior to the curing phase,thereby providing a better bonding with the concrete.

The bars according to the present invention may be mixed with greenconcrete and used for repair work of cracked concrete, also forproviding average residual strength and increased flexural strength inthe cured concrete structures, thereby restoring or improving thestructural integrity of the concrete structure.

Possible other areas of use are concrete floors in buildings, eitherprefabricated or in situ concreted; concrete paving stones which may bemade thinner and lighter due to the strengthening effects of the basaltMiniBars™, etc. Another area of use is as concrete for producing clampsor weights holding sea pipelines down on the sea bed.

Other type of use of the MiniBars™ according to the present inventionmay for example, but not exclusively, be very suitable for use onstructures that are exposed to liquids and in particular to water havinga ph below seven or water containing salt. Such structures may forexample be structures for sea defense and portions of jetties/quay wallsbelow or exposed to a waterline, pillars for bridges, concrete barges orthe like. The reinforcement may also be used on land based structureswhere access to install conventional reinforcement is difficult. Suchapplication may for example be deep foundations in excavations ordiaphragm wall, piles, or the like.

It should be noted that the basalt MiniBar™ reinforcement may be addedto the green concrete during mixing, delivered by trucks. Alternativelythe MiniBar™ reinforcement may be delivered in dry concrete for pavementstones and drainage culverts, etc.

The material used for establishing the helical pattern of the bars mayfor example be an elastic or in-elastic thread. As an alternative,basalt fiber threads may also be used since such helix also maycontribute to both strength and stiffness of the MiniBars™.

Further, it should also be appreciated that the MiniBars™ in additionmay be coated with a layer of randomly arranged particulate material,such as sand, glass or similar type of hard materials.

According to the present invention the MiniBars™ are evenly mixed in thegreen concrete, randomly orientated. The MiniBars™ have a similardensity to the concrete, although not exactly the same. Consequently theMiniBars™ do not float up nor sink in the green concrete and withoutbeing affected by vibrating of the concrete, i.e. neither migrating upto the top of or down to the bottom of the green concrete when theconcrete is vibrated.

The behavior of the MiniBars™ in the concrete is considered to bedependent on both the concrete properties and the distribution of theMiniBars™ in the concrete. The concrete properties may be importantbecause the bars are short compared to their diameter, and so do notdevelop a full anchoring in the concrete. Therefore, the forces that canbe mobilized in the bars are very dependent on the concrete strength andthe resulting bonding stress developed between the concrete and thebars. Distribution of the MiniBar™ in the concrete is important becauserelatively small number of bars are used in the mix, compared withconventional fibers. This relatively small number of bars means thatminor variation in distribution through the mix could have a notableeffect on the strength.

Further, the size of the aggregates used in the concrete mix may have aneffect on the strength of the cured concrete structure. Smalleraggregate size mixed with the MiniBars™ according to the presentinvention have affected the quality of the bar distribution andconsequently improved the concrete strength.

According to the present invention the helix around the straight fiberbundle may be beneficial. More or less randomly positioned MiniBars™according to the present invention will act like shear links in theconcrete structure, bridging and improving the shear strength of theconcrete. The MiniBars™ according to the present invention may also be asupplement conventional reinforcement, either conventional longitudinalflexural steel or basalt or carbon fiber reinforcement bars or cages,the MiniBars™ functioning at least as shear reinforcement, for exampleto reduce the required fixing time by the reinforcement fixers.

A unique advantage obtained by use of the MiniBars™ according to thepresent invention, is that tests has demonstrated that the relevantresidual strength requirements, based on ASTM C1609 tests (as specifiedin ACI 318-08 for steel fiber reinforced concrete) for using theMiniBars™ according to the present invention as shear reinforcement inreinforced concrete slabs and beams. Such type of fibers are of acorrosion free, alkaline resistant, structural fiber type.

The basalt fiber reinforcement bars according to the present inventionhave the following bonding mechanisms:

-   -   On the macro scale, the controlled pitch of the basalt fiber and        the helix thread twist in a range of 10 to 22 mm. The bonding        will be between the aggregates of the concrete, such aggregate        having irregular shape which will hook or create a friction        and/or mechanical bond with the indents in the mini bar surface        and with the other surrounding aggregates in the concrete,        securing a proper bonding effect. In addition the fine sand        particles and the cement particles lying in between the larger        aggregates will also contribute to this bonding effect. If the        pitch length of the mini bars according to the present        invention, i.e. the distance or length of one turn of the thin        helical string, is too large and/or to straight, i.e. very large        pitch, the MiniBars™ will be pulled out, while if said distance        or length is too small, the minibar according to the present        invention will breaks and/or crush fine particles surrounding,        adjacent cement, such particles mainly being fine particles due        to the reduces volume of indents per length of the bar.    -   On the micro scale, the surfaces of the discrete basalt fibers        will be roughened due to the tiny longitudinal indents formed        between the parallel fibers in the bundle, forming a bonding        effect between the fine particles in the concrete, allowing and        providing strong interlocking micro bonding effect between the        small aggregate and fines in concrete and the MiniBar™.

One feature of the RFT process is to be able to match the helix pitchlength (see FIG. 3 ) to fit the largest aggregate size such that theMiniBar™ and aggregate can interlock in the most efficient manner, ie,smaller pitch lengths to match smaller aggregate mixes.

The chemical bond of the concrete with the thin layer of the matrix andoutermost strands of the basalt fiber will also contribute to thebonding effect between the fibers and the surrounding concrete.

The above bonds are directly with the straight basalt fibers with smalltwist encased and joined by matrix. The bond does not rely on theaddition of sand particles which has been shown to shear off the vinylester coated bars. Further, the bond does not rely on a bond with anexternally added and “glued”-on ring of secondary material as proposedin the prior art. The MiniBar™ bond is in the direction of the fibers,and both the fibers and the indents made by the helically twisted thinthread allow for a good mechanical linkage between the reinforcement barand the surrounding concrete over the entire length of the MiniBar™.

It should be appreciated that in order to provide the roughened surfaceof the MiniBars™ according to the present invention, the weight factorof the fibers in respect to the weight factor of the matrix shouldpreferably be in the range of 65 to 85, more preferably in the order of70 to 77, and most preferably around 75. If the weight factor of thematrix used is too high, the fine indents between the fibers at thesurface of the Minibar™ will be filled with matrix, thus reducing thecontribution of the aggregate/fines to the micro scale bonding andcausing the matrix to be easily pulled off as a “hose”. If the volume ofthe matrix is too small, the shear contribution provided by the bondingbetween the fibers at the surface and the aggregates and/or fines in theconcrete, will be reduced.

Further, the most preferred angle α of the helix with respect to thecenterline of the MiniBar™ according to the present invention shouldpreferably be in the region of 4 to 8 degrees, while the angle x of theparallel fibers with respect to said centerline of the MiniBar™ shouldpreferably be in the order of 2 to 5 degrees. The Minibar™ maypreferably be produced according to the content of U.S. Pat. No.7,396,496, the content of which hereby is incorporated by the reference.Tests have proved that the fibres according to the present inventionmixes well and stay random in the mix regardless of the spinning speedof the rotating drum of the concrete mixing transport truck. Further,the fibres stay randomly distributed and stay evenly distributedthroughout the mixed volume also during pouring.

It should also be appreciated that both the diameter and the bondstrength is critical for securing the required strength of the minifibre reinforcement.

While the prior art solutions rely on the shear strength of the epoxyused as matrix, the fiber bars according to the present invention relyon the shear strength between the sand and the aggregates in theconcrete on the one side and the obtained bonding with the surface ofthe mini bar surface.

The range of diameters is important as the shrinkage in concrete alsoacts as a clamping mechanism which is stronger on the larger diametersthan the small diameters. Testing has shown that as the diameter isreduced the efficiency in clamping as measured as bond in the FlexuralTensile Testing increases, whereas the bond as measured by AverageResidual Strength decreases. Implications are that for differentstrength levels as required during engineering of concrete structuresdifferent diameters may be specified to provide the strength leveldesired or required.

Compared to the dimensions of the MiniBars™ the aggregate may have anynormal size commonly used in concrete.

SHORT DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described in further details,referring to the accompanying drawings, wherein:

FIG. 1 shows schematically a view of a first embodiment of a MiniBar™according to present invention, indicating a tight wind;

FIG. 2 shows schematically a view of a second embodiment of a MiniBar™according to the present invention, showing windings having longer pitchlength;

FIG. 3 shows schematically and in an enlarged scale a portion of oneembodiment of a MiniBar™ according to the present invention, indicatingvarious angles of importance;

FIG. 4 shows schematically in an enlarged scale a vertical section inaxial direction of an embodiment of a MiniBar™ according to the presentinvention, indication the direction of the numerous substantiallyparallel fibers and indicating the interaction between the aggregatesand fines of the concrete on the one hand and the surface and indents ofthe MiniBar™ fiber surface on the other hand;

FIG. 5 shows schematically in an enlarge scale a cross section through aMiniBar™ according to the present application, indication also theindents and the roughened surface;

FIG. 6 shows a graph showing the flexural tensile strength measured inMPa of a dry mix concrete for various fiber dosages by volume %;

FIG. 7 shows average residual strength measured in MPa for a dry mix ofvarious fiber dosages by volume %; and

FIG. 8 shows the flexural tensile strength measured in MPa, of normalconcrete with 20 mm maximum aggregate size, for different fiber dosagesby volume %;

FIG. 9 shows flexural tensile strength of high strength concrete with 20mm maximum size aggregate, for different fiber dosages by volume %;

FIG. 10 shows average residual strength concrete with 20 mm maximum sizeaggregate; and

FIGS. 11A-11C are presented on one sheet disclosing the results fromtests, shown in FIG. 11A (Table 1), (Table 2), and FIG. 11C (Table 3),where FIG. 11A (Table 1) discloses the test results for generation 1 and2 of dry mix concrete; FIG. 11B (Table 2) shows the test results fornormal concrete with maximum 20 mm aggregates, the dosage % being thevariable; and FIG. 11C (Table 3) shows the test results for highstrength concrete with maximum 20 mm aggregate for three different fiberdosage %.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematically a view of a first embodiment of a MiniBar™ 10according to present invention. The MiniBar™ 10 comprises a large numberof parallel fibers 11 of basalt, glass fiber or carbon, embedded in acured matrix of a conventional type resisting alkaline attacks. Suchmatrix may for example be a thermoplastics, a vinyl ester (VE) or anepoxy. A string elastic or string inelastic 12 is wound continuouslyaround the elongate embedded fibers, applying a certain tension in thestring 12 so as to partly deform the circumferential surface of the bar10, producing elongate helically arranged indents 14. This windingoperation is preferably performed simultaneously with or slightly afterthe embedment process of the elongate fibers 11 in the matrix, but priorto the final stage of the curing thereby securing the requireddeformation of the circumferential surface of the bars 10. Further, theMiniBar™ 10 may be made as elongate strings or bars in a continuousprocess, whereupon said continuous bar is cut into lengths preferably inthe range 20 mm to 200 mm, while the diameter or thickness of the barsmay preferably be in the range of 0.3 mm to 3 mm. The helical may bemade of an elastic or inelastic string for example of basalt which, whentensioned in a controlled manner, can create the repeatable and desiredsurface deformation in the form of indents. Further, the externalsurface of the MiniBar™ may preferably have a hair like texture,comprising a number of fine, hairs or fiber ends extending out from theMiniBar™ in a random direction. This may be achieved by twisting thelarge number of parallel basalt fibers embedded in an uncured matrix,preferably as a single bundle, around said fine helical, thustransforming straight fine thread to a helical around the fiber bundle.During the process of establishing the helical, the tension in the fine,thinner helical is controlled with respect to the tension in the basaltfiber bundle. The embodiment shown in FIG. 1 is the primary means forenhancing the bonding with the surrounding concrete is the uneven shapeof the MiniBar™ formed by the tensioned helical 12. The difference intension is maintained in the bar until the matrix is sufficiently curedand hardened. A secondary means is the bonding to the concrete at themicroscopic level with the rough surface created by the fibersprotruding partly from the matrix.

FIG. 2 shows schematically a view of a second embodiment of a MiniBar™10 according to the present invention. According to this embodiment theMiniBar™ 10 is provided with a helical 12 as shown in FIG. 1 . Inaddition the two ends 13 are deformed/flattened so as to increase theend contact area, thereby enhancing the bonding properties and shearresistance capacity of the MiniBar™ 10 with respect to the surroundingconcrete. Although a helical 12 is shown, it should be appreciated thatthe MiniBar™ 10 may be without such helical 12, the deformed orflattened ends securing the required bonding and shear resistancecapacity, ref FIG. 3 , showing schematically a view of a thirdembodiment of a MiniBar™ 10 according to the present invention, deformedat each end and without a helical 12.

FIG. 3 shows schematically and in an enlarged scale a portion of oneembodiment of a MiniBar™ according the present invention, indicatingvarious angles of importance. As shown the bar 10 comprises a largenumber of substantially parallel fibers 17, embedded in a suitablematrix, the bar 10 being provided with a helically wound string 12,tensioned so that the helical string 12 forms elongate helicallyextending indents 14 along the length of the bar 10. As indicated in theFigure, an angle α is used to define the angle between the centerline CLof the bar 10 and the projected angle of the helix 12 in the paperplane. Such angle α should preferably be in the range between 4 and 8degrees. Further, the Figure also shows the angle β between thecenterline CL of the rod and the longitudinal direction of the fibers17. As specified above the angle β should be in the region between 2 and5 degrees. The optimum is a balance in tension between both fibers and acommon angle of 4 to 5 degrees to the centerline for both fibers. Itshould be appreciated that the FIG. 3 is exaggerated and distorted so asto indicate the various forms emanating from the tensioned helix. Itshould be appreciated that the surface between the helix is slightlygiven a helically arranged convex outer surface. The length L betweentwo consecutive indent points in the axial direction of the bar definesthe pitch length of the helix.

FIG. 4 shows schematically in an enlarged scale a vertical section inaxial direction of an embodiment of a MiniBar™ 10 according to thepresent invention, indicating the direction and path of the numerous,substantially parallel fibers 17 and also indicating the interactionbetween the aggregrates 16 and fines of the concrete 15 on the one handand the surface and indents 14 of the MiniBar™ fiber surface on theother hand. It should be appreciated that from a clarity point of viewonly a part of the surrounding concrete 15 is shown, the fibers 10 beingrandomly arranged in the concrete.

FIG. 5 shows schematically in an enlarge scale a cross section through aMiniBar™ 10 according to the present application, indication also theindents 14, the helix 12 and the roughened surface of the bar 10. Itshould be appreciated that the roughened surface is established by theparallel fibers 17 and elongate small indents between adjacent fibers17.

Normally, the range for adding crack control products is less than 2%,while according to the present invention the range of added dosage ofMiniBars™ is in the range of 0.5% to 10%. Test have shown that usingMiniBar™ reinforced concrete within the above identified range of addedMiniBars™, demonstrated no difficulty in concrete mixing. There was nobleeding, balling or segregation in the concrete, demonstrating that itis feasible to mix MiniBars™ in concrete without any difficulty. Testhave proved that such concrete was handled, placed, consolidate andfinished normally without additional precautions, thus demonstratingthat good workability can be achieved due to the density of MiniBars™.

Tests have been performed to validate and verify the improvements to theconcrete. The tests showed that compressive strength according to ASTMC39ASTM C39 of cylinders reinforced with MiniBar™ reinforced concreteaccording to the present invention, demonstrated ductile failure withthe cylinders still intact after failure while normal non-reinforcedcylinders would shatter due to brittle failure.

FIG. 6 shows a graph showing the flexural tensile strength measured inMPa of a dry mix concrete for various fiber dosages by volume %. Thegraph shows the testing of two generation fibers in a dry mix. The maindifference between the two generations fibers are the fiber diameter andthe pitch length of the helix. In the first generation the fiber dosageby volume was constant, i.e. 1.89 volume %, while in the Gen. 2 thefiber dosages were 0.75 and 1.5 respectively. As shown, the residualstrength for both Gen 2 was higher then the corresponding results forGen 1, in spite of a reduction in fiber dosage due to efficient use ofmaterials and the high tensile strength of the basalt.

FIG. 7 shows average residual strength measured in MPa for a dry mixconcrete using various fiber dosages by volume %. The low averageresidual strength is the result of fewer MiniBars™ across a given crackface.

FIG. 8 shows the flexural tensile strength measured in MPa, of normalconcrete with 20 mm maximum aggregate size, for different fiber dosagesby volume %, varying from 2 to 10 volume % and a more or less linearincrease in flexural tensile strength for increasing volume percentages

FIG. 9 shows flexural tensile strength of high strength concrete with 20mm maximum size aggregate, for different fiber dosages by volume %,varying from 0.5 to 10.0, a 17.04 MPa flexural strength being achievedwhen using a dosage of 10 volume %. Correspondingly, FIG. 10 showsaverage residual strength concrete with 20 mm maximum size aggregate,obtaining an average residual strength of 15.24 when using a fiberdosage of 10.0 volume %.

The Figures also include one sheet disclosing the results from tests,shown in Table 1, table 2 and Table 3. Table 1 discloses the testresults for generation 1 and 2 of dry mix concrete; Table 2 shows thetest results for normal concrete with maximum 20 mm aggregates, thedosage % being the variable; and Table 3 shows the test results for highstrength concrete with maximum 20 mm aggregate for three different fiberdosage %.

The flexural tensile strength (modulus of rupture) was tested per ASTMC78-07 for MiniBars™ according to the present invention in volumepercentages from 0.75% up to 10% with results in flexural tensilestrength increasing from 6 MPa up to 17.05 MPa depending on volumefraction used over a zero MiniBar™ result of 5.2 MPa.

The average residual strength increased from zero for normalun-reinforced concrete up to 5.8 to 15.24 MPa, (474 psi to 1,355 psi),depending on volume fraction of MiniBars™ used. These values aresignificantly greater than those expected for plain concrete of similarcompressive strength. The following correlation between flexural tensilestrength (f_(r)), MiniBar™ dosage by volume (V_(f)) and (f′_(c)) is thecompressive strength of concrete, determined by using standard cylindertests for (all units being MPa units):f _(r)=(0.62+0.076 V _(f))√{square root over (f′ _(c))}

The average residual strengths (ARS) obtained for MiniBar™ reinforcedconcrete according to the present invention were much greater thanexpected, suggesting that the MiniBar™ have significantly helped in thepost-cracking performance of concrete in the current test program.

The Average Residual Strength ARS=1.95 V_(f), where V_(f) is theMiniBar™ dosage in percent by volume and f′_(f) is the concretecompressive strength.

In order to improve the bonding between the MiniBars™ and the concretein which the MiniBars™ are embedded, the surface of the MiniBars™ may beprovided with a randomly arranged particulate material, such as forexample sand. It should also be appreciated that the MiniBar™ may beprovided with a longitudinal opening extending axially through theMiniBar™ thus securing a tubular MiniBars™ to increase bond area. Itshould also be appreciated that the MiniBar™ is thicker thanconventional steel or plastic material fibres used and is suited toexperience higher compression forces, due to concrete shrinkage on alarger diameter.

The specific gravity p of steel is in the order of 8 g/cm³, while thespecific gravity p for concrete is around 2.3. The specific gravity ofthe MiniBar™ reinforcement is in the region 1.9. As a consequence, theMiniBar™ does not sink nor float up towards the surface of the concretemix during casting or concreting, since the specific gravity of thebasalt fibres corresponds more or less to the aggregates used in theconcrete.

The process for manufacturing the MiniBars™ according to the presentinvention, comprises the following steps:

-   -   A number of continuous basalt fibers are assembled in parallel        and embedded in a matrix of vinyl ester. During this phase, the        fiber bundle is pulled forward, subjected to a pulling tension,        forming a straight body, the matrix still being uncured and        soft. The fibers are delivered from reels into a wetting        chamber.    -   One or more separate strings are helically wound around the        straight, matrix embedded bundle while the bundle and matrix        still are relatively soft, said one or more separate strings        being subjected to a higher tension than the tension caused by        the pulling forward of the matrixed fiber bundle. Due to said        higher tension, said one or more separate strings will formed        helically extending indents in the surface of the matrix        embedded fiber bundles.

Thereupon, the matrix embedded bundle and said one or more helicallywound, more or less embedded strings enter a curing stage where thefiber bundle with its helical string(s) are cured and hardened.

Due to said higher tension in said one or more strings, compared to thetension pulling the fiber bundle forwards, the straight shape of fiberbundle will also be affected, obtaining a more or less helical overallshape prior to and during the curing stage.

-   -   The elongate fiber bundle is then chopped into units having the        required length specified above, and bagged, suitable for use.

It should be appreciated that the pitch given to the fiber bundle, andhence the MiniBars™ is dependent upon the difference in tension betweenthe tension in said one or more thin strings during winding and thetension applied for pulling the fiber bundle forward during the windingprocess. The higher tension in said one or more thin strings compared tothat of the fiber bundle, the shorter pitch and deeper helical indents.

The invention claimed is:
 1. A set of reinforcement bars forreinforcement of concrete structures, comprising: a plurality ofreinforcement bars that are configured to be mixed together in randomorientations with green concrete, wherein the reinforcement bars eachcomprise at least one fiber bundle comprising a plurality ofsubstantially parallel fibers, being made of basalt, carbon, or glassfiber, and being embedded in a cured matrix, wherein each of the fibersof each of the at least one fiber bundle has a cylindrical shape and across section, the cross section being circular or oval, and one or morestrings of an elastic or inelastic, tensioned material helically woundaround the at least one fiber bundle of the fibers prior to a curingstage of the matrix in which the at least one fiber bundle is embedded,wherein the one or more strings maintain the fibers in a substantiallyparallel state during the curing stage and deform at least a portion ofan external surface of the at least one fiber bundle prior to or duringthe curing stage of the matrix to such that the one or more strings formlongitudinally-arranged helical indents extending in a longitudinaldirection along the length of the bar and the external surface of the atleast one fiber bundle is an uneven external surface due to the helicalindents formed by the one or more strings, wherein the reinforcementbars each have a length in the range of 20 mm to 200 mm, and a diameterin the range of 0.3 mm to 3 mm, wherein the reinforcement bars each havea roughened surface shape and/or texture which contributes to bondingwith the concrete structures when in a hardened or cured state, whereina higher tension is applied to the string than the bundle, therebyproviding a twist in the bundle.
 2. The set of reinforcement barsaccording to claim 1, wherein the one or more strings of each of theplurality of reinforcement bars comprise two or more strings that arehelically wound in opposite directions around the at least one fiberbundle of each of the plurality of reinforcement bars.
 3. The set ofreinforcement bars according to claim 1, wherein, for each of theplurality of reinforcement bars, the helical indents formed by the oneor more strings have a pitch length in the range of 10 mm to 22 mm. 4.The set of reinforcement bars according to claim 3, wherein, for each ofthe plurality of reinforcement bars, the pitch length of the helicalindents formed by the one or more strings is 17 mm.
 5. A concretestructure comprising: concrete having a grade of concrete and aggregatesize; and the set of reinforcement bars according to claim 3, whereinthe pitch length of the helical indents is matched with the grade ofconcrete and aggregate size.
 6. The set of reinforcement bars accordingto claim 1, wherein the roughened surface shape and/or texture comprisesthe helical indents, wherein the helical indents are configured to forma bonding effect with the concrete structures.
 7. The set ofreinforcement bars according to claim 1, wherein, for each of theplurality of reinforcement bars, the helical indents formed by the oneor more strings extend circumferentially continuously along the entirelength of the bar.
 8. The set of reinforcement bars according to claim1, wherein the fibers are continuous along the length of each of thereinforcement bars.
 9. The set of reinforcement bars according to claim1, wherein the one or more strings are under a higher tension than theat least one fiber bundle in the curing stage and until the matrix iscured and hardened.
 10. A green concrete mixture comprising: greenconcrete; and the set of reinforcement bars according to claim 1,wherein the plurality of reinforcement bars are evenly mixed andrandomly orientated in the concrete.